It is well known to use bio-active materials to coat structures to be introduced into a living system. Over the last 30 years, research into this area has become increasingly important with the development of various bio-compatible articles for use in contact with blood, such as, for example, vascular grafts, artificial organs, endoscopes, cannulas, and the like.
While various materials have been used to make such articles, synthetic polymers have been increasingly popular as the preferred materials due to their antithrombogenic and good mechanical properties. For example, polyurethane is a useful and effective material with a variety of clinical applications. Although synthetic polymers, such as, PTFE and polyurethane, are less thrombogenic than earlier materials, thrombus formation is still a problem.
A thrombus is the formation of a solid body composed of elements of the blood, e.g., platelets, fibrin, red blood cells, and leukocytes. Thrombus formation is caused by blood coagulation and platelet adhesion to, and platelet activation on, foreign substances. When this occurs, a graft is occluded by such thrombogenic material, which in turn, results in decreased patency for the graft. Accordingly, more stringent selection criteria are necessary for small caliber vascular graft materials because the small diameters of these grafts magnify the problem of deposition of such thrombogenic material on the luminal surfaces of the graft. Thus, thrombus formation is a serious complication in surgery and clinical application of small caliber vascular grafts.
Various anti-thrombogenic agents, such as heparin, have been developed and incorporated into bio-compatible articles to combat thrombus formation. In a living system, heparin inhibits the conversion of a pro-enzyme (prothrombin) to its active form (thrombin). Thrombin catalyzes a complicated biochemical cascade which ultimately leads to the formation of a thrombus.
Infection is also a serious concern for articles to be implanted into a host organism. Bacterial, viral and other forms of infection may lead to life-threatening complications when an article is implanted into a host organism. Thus, binding of an anti-infection agent to a surface of an implantable article can reduce the risk of infection when such an article is introduced into a host organism.
The art is replete with various procedures for preventing thrombus formation and/or infection by modifying polymeric surfaces. Various substrate surfaces have previously been described that are suitable for introducing into a biological system. For example, bio-compatible polymer surfaces have been described with various benefits including decreased thrombogenicity, increased abrasion-resistance and improved hydrophilic lubricious properties. Additionally, U.S. Pat. No. 5,061,777 describes procedures for modifying polyurethanes and polyurethaneureas in order to decrease their thrombogenicity. Similarly, U.S. Pat. No. 5,077,352 describes a method of forming a polyurethane complexed with a poly(ethylene oxide) having good adherence to a substrate and good anti-friction properties.
These polymer surfaces, however, are not completely bio-compatible. Thrombus formation and infection continue to pose problems when such articles are implanted within a host. These articles especially are not suitable for use with small caliber vascular grafts where graft patency is critical. Thus, procedures for grafting bio-active agents onto a substrate surface have been developed.
For example, bio-active agents directly bound to the polymer backbone of a polymer coating material are known. Hu et al. in U.S. Pat. No. 5,077,372 disclose a medical device coated with an anti-thrombogenic agent, e.g., heparin, covalently linked to the amino groups of the polyurethane coating. These coating reactions and heparinizations are carried out directly on the device's surface. Such methods, however, suffer from decreased bio-activity, and consequently, increased thrombogenicity because the bio-active agent, such as, heparin, must be positioned at a distance from the substrate surface in order to optimally interact with its physiological substrates.
Accordingly, alternative methods have been developed for binding bio-active molecules to substrate surfaces. In particular, methods for ionically binding bio-active agents to a substrate via a quarternary ammonium compound have been described. See for example, Mano in U.S. Pat. No. 4,229,838; Williams et al. in U.S. Pat. No. 4,613,517; McGary et al. in U.S. Pat. No. 4,678, 660; Solomon et al. in U.S. Pat. No. 4,713,402; and Solomon et al. in U.S. Pat. No. 5,451,424.
These methods, however, are severely limited because the bio-active agent is leached over time from the surface of the substrate. Thus, the protection afforded by the ionically bound bio-active agent is transient at best. Such procedures, therefore, also are not suitable for small caliber grafts.
Accordingly, more permanent methods for binding bio-active molecules to substrate surfaces have also been developed. These methods include covalently binding a bio-active molecule, either directly, or via a spacer molecule, to a substrate surface. For example, photochemical reactions are described which covalently bind bio-active agents to substrate surfaces. See U.S. Pat. Nos. 4,331,697; 4,973,493; 4,979,959; and 5,258,041. When photochemical reactions are used to covalently bind bio-active agents to substrates, however, the choice of substrate is limited. Actinic radiation causes certain substrates, for example PTFE, to degrade. Thus, these methods are limited by the substrate material to be coated.
Even though photochemical reactions are limited to use with certain actinic radiation-resistant substrates, these reactions have been used to indirectly bind bio-active coatings to such substrates via a spacer molecule. For example, several studies describe polyurethane coatings having various spacer molecules that link bio-active agents to polymer substrates. These studies indicate that bio-active agents, such as, for example, heparin, bound to polymer coatings retain more of their bio-activity if they are tethered away from the surface of an article by a spacer.
Thus, Bichon et al. in U.S. Pat. No. 4,987,181 describe a substrate having an adhesive film with anti-thrombogenic properties on its surface. This adhesive film is an olefinic copolymer having carboxylic side chains of the formula O.dbd.CH--NH.sub.2 --(CH.sub.2).sub.n --NH.sub.2 --CH.sub.2 --R, wherein R is a heparin molecule or a depolymerization fragment of a heparin molecule. The adhesive film is deposited onto the substrate via photo-initiated polymerization of a suitable monomer. Thus, heparin, or a fragment thereof, is covalently linked to the substrate via an amine spacer.
Although covalent bonding of the bio-active agent to the substrate surface with, or without, a spacer molecule therebetween solves certain problems in the art, these methods continue to be limited. In particular, certain bio-active coatings begin to degrade in response to the photochemical signals used to bind them to the substrate surfaces. In a similar fashion, certain polymeric substrates, such as, polytetrafluoroethylene, degrade when exposed to photochemical reactions and are therefore not useful with such coatings. Thus, attempts have been made to use spacer molecules to bind bio-active agents to substrate surfaces without photochemical reactive groups.
For example, in a four step process, Park et al. disclose immobilizing heparin onto a commercial preparation of a segmented polyetherurethaneurea (PUU) using hydrophilic poly(ethylene oxide) (PEO) spacers of different molecular weights. Their method includes (1) coupling hexamethyldiisocyanate (HMDI) to a segmented polyurethaneurea backbone through an allophanate/biuret reaction between the urethane/urea-nitrogen proton and one of the isocyanate groups on the HMDI. Next, (2) the free isocyanate groups attached to the backbone are then coupled to a terminal hydroxyl group on a PEO to form a PUU-PEO complex. Next (3) the free hydroxyl groups of the PUU-PEO complex are treated with HMDI to introduce a terminal isocyanate group. Finally, (4) the NCO functionalized PUU-PEO is then covalently bonded to reactive functional groups on heparin (--OH and --NH.sub.2) producing a PUU-PEO-Hep product. K. D. Park and S. W. Kim, "PEO-Modified Surfaces-In Vitro, Ex Vivo and In Vivo Blood Compatibility", in Poly(Ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications 283 (J. Milton Harris ed. 1992). This method will be referred to hereinafter as the "Park Method."
Although the use of spacer molecules to tether bio-active molecules to substrate surfaces increases the anti-thrombogenicity of certain substrate surfaces, problems still arise in applying such coatings to chemically inert substrates, such as PTFE, PET and the like. These coatings, which have hydrophilic properties adhere weakly, if at all, to hydrophobic chemically inert substrate surfaces. Thus, the natural repulsive forces between the hydrophilic coatings and the hydrophobic substrate surface serves to decrease the ability of the coating to remain secured to the substrate surface. Thus, plasma treatment of substrate surfaces has been developed as a method to alter the surface properties of such substrates in order to secure coatings thereto.
Accordingly, surfaces of chemically inert tubes have been modified in order to promote binding between a coating and the substrate surface by deposition of a thin layer of an appropriate polymer onto the substrate surface using plasma polymerization (also known as glow discharge) techniques. This technique involves introducing a polymerizable organic monomer in a gaseous state into a vacuum container together with the substrate material to be coated. The gas is then subjected to an electric discharge which initiates a polymerization reaction. This reaction generates ions or free radicals which react with and deposit on the substrate. The polymer formed is normally deposited as a thin layer over the substrate material present in the reaction vessel. Critically, the bulk substrate characteristics are preserved, but the surface properties, which are major determinants of bio-compatibility and non-thrombogenicity, can be modified or improved by plasma polymerization.
Accordingly, Hu et al. in U.S. Pat. No. 4,720,512 describe a method for imparting improved anti-thrombogenic activity to a polymeric support structure by coating it with an amine-rich material, e.g., polyurethaneurea, introducing hydrophobic groups into the amine-rich surface coating through plasma treatment with fluorine compounds, and covalently bonding an anti-thrombogenic agent to the hydrophobic amine-rich surface. Similarly, Hu et al. in U.S. Pat. No. 4,786,556 describe substituting siloxane and silazane compounds during the plasma treatment step of the '512 patent for the previously disclosed fluorine compounds. See also, Narayanan et al. in U.S. Pat. Nos. 5,132,108 and 5,409,696 and Feijen et al. in U.S. Pat. No. 5,134,192 for other examples of plasma treating substrates prior to introduction of a bio-active molecule.
These preceding methods for plasma treating a substrate surface are limited in their scope because they only work with certain substrates. Thus, they do not provide a general purpose coating composition that can bind to a variety of substrate surfaces.
All of these disclosures have addressed substrate surfaces and/or coatings therefor which can exist within biological systems and, in particular, can increase the anti-thrombogenicity of the surface of, e.g., medical articles. These reactions, however, cannot be universally applied to substrate surfaces. Accordingly, when chemically inert ePTFE vascular grafts are desired to be used, they must be implanted without the benefit of, e.g., an anti-thrombogenic coating, or with one of the previously described coatings. In either case, the patency of the graft is severely compromised because foreign bodies build up on and occlude the graft. This is especially problematic in small caliber grafts where the diameter of the lumen is smaller than other larger diameter vascular grafts. Thus, there is a need for a chemically inert vascular graft material with increased patency and methods of making such a graft. In particular, there is a need for a chemically inert small caliber vascular graft having a surface that adheres well to bio-active coatings applied thereto. The present invention is directed toward providing such a solution.